Determination of elastic and resistive properties of the respiratory system is of considerable importance in monitoring disease progression regardless of ventilator mode used. In the proportional assist mode of ventilation, knowledge of these properties is, in addition, essential for proper adjustment of the volume-related and flow-related assist gains. The proportional assist mode of ventilation (PAV.RTM.) is fully described in U.S. Pat. No. 5,107,830 (Younes), the disclosure of which is incorporated herein by reference.
Briefly, in proportional assist ventilation, the pressure delivered by the ventilator increases in direct proportion to patient effort and the proportionality applies from breath to breath as well as continuously throughout each inspiration. Proportional assist ventilation operates on the principle that the inspiratory flow (V) and its integral, volume (V) of a patient contain the necessary information to substantially match the profile of patient effort. By continuously measuring the instantaneous values of flow and volume and applying a gain factor appropriate to each (for flow, cmH.sub.2 O/L/S and for volume cmH.sub.2 O/ml), the ventilator can deliver a pressure profile to the patient that amplifies the instantaneous pressure generated in the patient.
The pressure assist provided to the patient may be expressed by the relationship: EQU P.sub.vent =K.sub.1 V+K.sub.2 V
where P.sub.vent is the magnitude of the pressure assist, K.sub.1 is a gain factor applied to a variable ongoing volume signal (V) and K.sub.2 is a gain factor applied to a variable ongoing flow signal (V). The K.sub.1 (or VA) and K.sub.2 (or PA) values are fractions of respiratory elastance and respiratory resistance respectively.
In apneic patients, the ventilator provides the only distending force, as reflected by airway pressure (P.sub.aw). Because P.sub.aw provides the total distending pressure, it is possible to reliably determine the elastic and resistive properties of such patients. A variety of reliable methods have been described in the literature for this purpose.
The situation is much more complex in the assist modes of ventilation where ventilator cycle is synchronized with patient's inspiratory effort. In this case, P.sub.aw is not the only distending force. Rather, flow (V) and volume (V) are generated as a result of the combined action of the ventilator (as reflected by P.sub.aw) and the patient (as expressed in muscle pressure (P.sub.mus)). To the extent that the magnitude of P.sub.mus is continuously varying and cannot be measured or estimated without prior knowledge of patient mechanics, it is not possible to estimate total applied pressure (i.e. P.sub.aw +P.sub.mus) at any instant of the inspiratory phase of the ventilator cycle. This problem has made it difficult to reliably estimate mechanical properties in the assist modes.
Given that the behaviours of P.sub.aw, flow and volume during a "normal" cycle cannot be used to estimate mechanical properties, one is left with the option of causing a perturbation in one of the primary variables (e.g. P.sub.aw or V) and observing the consequences on the other variables. This approach, which has been used successfully in the controlled ventilation modes, is fraught with difficulties in the assist modes, since the perturbation produced at the airway may alter the pressure generated by the patient (P.sub.mus) via at least three mechanisms, namely:
(a) Mechanical perturbations are readily perceived. The patient may react at a behavioural level, altering P.sub.mus ; PA1 b) The change in flow or volume produced by the perturbation may alter P.sub.mus reflexly (i.e. independent of perception); and PA1 c) The change in flow or volume may alter P.sub.mus at a strictly mechanical level via the intrinsic properties of respiratory muscles (force-length and force-velocity relations). PA1 a) placing a ventilator in the proportional assist mode of ventilation, using empiric values of elastance and resistance or values of elastance and resistance determined by other conventional methods, for initial adjustment of the volume-related and flow-related assist components of the proportional assist ventilation, PA1 b) monitoring airway pressure (P.sub.aw) and flow (V) and volume (V) to the patient, PA1 c) holding flow at or near zero in selected breaths for a period beyond termination of an inspiratory phase of the ventilation, PA1 d) measuring P.sub.aw at a point as far away as possible from the onset of the inspiratory hold but sooner than the latency for behavioral respiratory responses to provide P.sub.hold, PA1 e) measuring the tidal volume (V.sub.T) of the breaths selected for the inspiratory hold step, PA1 f) establishing the relationship between P.sub.hold and V.sub.T in said selected breaths to provide a pressure-volume relationship over the V.sub.T range encountered during proportional assist ventilation to permit subsequent adjustment of the volume-related assist for proportional assist ventilation. PA1 a) placing a ventilator in the proportional assist ventilation mode using empiric values of elastance and resistance or values of elastance and resistance determined by other conventional methods, for initial adjustment of the volume-related and flow-related assist components of the proportional assist ventilation, PA1 b) monitoring airway pressure (P.sub.aw) and flow (V) and volume (V) to the patient, PA1 c) applying brief perturbations in pressure, flow and volume of at least two different forms to selected breaths, such perturbations occurring at a predetermined time during the inspiratory phase of said selected breaths, PA1 d) determining the P.sub.aw, flow (V) and volume (V) at predetermined times during the perturbation less than latency for behavioural responses, PA1 e) determining the P.sub.aw, V and V at similar times to those used in step (d) in unperturbed breaths, PA1 f) averaging the results of a number of each form of perturbation and of unperturbed breaths, and PA1 g) utilizing the average values of P.sub.aw, V and V obtained from the different forms of perturbation and from unperturbed breaths to provide the pressure-flow relationship to permit subsequent adjustment of the flow-related assist for proportional assist ventilation.
If P.sub.mus is altered by the perturbation, then one cannot assume that the change in P.sub.aw during the perturbation represents the total change in applied force which, in the assist mode, is given by .tangle-solidup.P.sub.aw +.tangle-solidup.P.sub.mus !. This relationship, again, makes it impossible to use the relation between P.sub.aw, V and V during the perturbation to estimate patient mechanics, unless it can be assured that the perturbation does not change P.sub.mus.